By the term “quantum dots”, reference is made here to nanometer sized particles of semiconductor material where quantum confinement is present. Depending upon the semiconductor material, the maximum sizes change but are usually below 100 nm. The exact size of the quantum dots may enable the semiconductor band gap to be modulated, which provides potential for increasing photo-electric conversion efficiency with respect to bulk films of semiconductor material in more conventional solar cells and related devices.
A three-dimensional ordered matrix of quantum dots (QDs), known as a QD solid (QDS) or a QD superlattice (QD-SL) has a potential in optoelectronic applications, including devices such as solar cells, LEDs, thermoelectric devices, etc. This type of structure is part of the “tandem” approach for developing new, third generation optoelectronic devices such as solar cells.
Hitherto QD-SLs have mostly been grown by epitaxial deposition techniques such as molecular beam epitaxy (MBE) or metal-organic chemical vapour deposition (MOCVD), these being techniques that require low vacuum, high temperature and pristine precursors. Solid state grown QD-SLs show a low defect density with QDs which are perfectly passivated by a bulk barrier material. On the other hand, their processing is characterized by high costs. Furthermore, only a few combinations of barrier material/QD pairs are able to be grown due to lattice matching constraints.
Alternatively, room temperature processing of colloidal QDs has been used for applying QDs to substrate surfaces. Colloidal QDs are most commonly spherical, but rod shapes and others are available. In this much cheaper approach, QD synthesis does not take place in situ (on top of a bulk substrate) but in solution. After QD synthesis, the QDs are applied to the substrate surface by spin coating or dropcasting protocols of molecularly passivated QDs onto a substrate. Due to the poorer surface passivation of the nanocrystals, the colloidal approach suffers from high defect concentration and is prone to photodegradation (through oxidation processes).
Meanwhile the technique of successive ion layer adsorption reaction (SILAR) is known for the preparation of thin films, as is disclosed in example in CN102251235 or CN101312218. In this technique, a substrate is alternatively soaked in a cation precursor solution and then an anion precursor solution. The cations may here for example the chosen among Cu, Zn, Sn and In, and the deposited anions are most commonly chalcogenides (sulfides and selenides in particular—tellurides are also possible candidates though not often used). More generally, thin films of CdS, CdSe, CdO, PbSe, PbS, SnS, ZnS, ZnO, and Fe2O3 have been reported as being prepared by SILAR techniques on a variety of substrates.